This invention relates to liquid phase sintered dense composite bodies of an alloy or alloys typified by cemented hard metals or cermets wherein refractory hard material such as carbides, nitrides, oxides, borides and silicides are cemented with metals and/or alloys and also to a method for manufacturing the same. More particularly, this invention relates to a dense sintered body of the type described which is formed with pores or grooves on a specific surface or surfaces thereof and a method for producing such dense sintered body. The metal referred to herein includes a pure metal or metals and an alloy or alloys.
A sintered body of a liquid phase sintered composite is produced by cementing hard refractory material with a metal or metals. Typical examples of such a liquid phase sintered body are a cemented hard metal produced by cementing its principal component, namely tungsten carbide with a cobalt metal and a cermet produced by cementing its principal component such as titanium carbide with a nickel metal. Generally, technical difficulties have been encountered in producing such a sintered body when the body has a complex shape, and such composite materials have also been expensive. Therefore, when cutting tools, wear resistant parts and machine parts of a complex shape or a large size are required, it has hitherto been customary to mechanically or physically affix, or to join by brazing, a small sintered body having a relatively simple shape to a base metal, such as steel, to thereby obtain the desired product.
Generally, a hard refractory material has a thermal expansion coefficient which is considerably lower than that of steel or other base metals. Accordingly, the thermal expansion coefficient of a sintered body is generally low and equal to or below one half that of steel. Thus, when a sintered body is joined by brazing to a base metal, stress and strain are likely to be generated in the interface betwen the sintered body and the base metal and in its vicinity, due to the difference in thermal expansion coefficient, and tensile stress is applied to the opposite surface of the sintered body. The stress and strain generated and applied by brazing lowers the strength of the sintered body, with the result that chipping or cracking tends to occur while the sintered body is being ground or placed in service.
In order to avoid or reduce the stress and strain generated by brazing and minimize any reduction in the strength of the sintered body, various proposals have been made. One such method consists of using a brazing alloy having a low melting point or of using a copper plate to effect sandwich brazing. However, no method which completely solves this problem has yet been developed. It is interesting to note that a TiC-Ni-Mo cermet is not used to produce a tool to be brazed, in spite of the fact that the cermets themselves can be used, as a cutting tool, in substantially the same applications as a cemented hard metal. The reason for this is that a reduction in strength that would be caused by brazing markedly reduces its adaptability as a material for producing a tool.
In producing a liquid phase sintered dense composite, it is a general practice to use a cementing metal in the form of a fine powder with a grain size of from less than 1 .mu.m to several .mu.m so as to facilitate densification of a compact body in the process of sintering and to impart optimum properties to the sintered body thus produced. After the cementing metal in powder form is uniformly mixed with hard refractory material, the mixture is molded into a compact body. In the process of heating the compact body to a sintering temperature and holding the same at the sintering temperature, the cementing metal is melted and the surface tension of the molten cementing metal causes the compact body to rapidly contract, thereby densifying the compact body. The transformation of the cementing metal into a liquid phase will be discussed more in detail. In the process of heating a compact body to a sintering temperature and holding the same at the sintering temperature, the elements constituting the hard refractory material which is in contact with the cementing metal first diffuse in the solid state into the cementing metal. This diffusion of the elements in the solid state into the cementing metal causes a change in the composition of the cementing metal and lowering of the melting point thereof. If the cementing metal forms a eutectic alloy with the diffused elements, then the cementing metal will melt when heated to a temperature above the eutectic temperature, thereby promoting densification of the compact body. This is a well-known fact.
In cemented hard metals of the WC-Co system, for example, the melting point of cobalt metal is 1495.degree. C. However, the eutectic temperature of the cementing metal of these cemented hard metals is about 1280.degree. C., so that sintering of the compact bodies of the mixture of hard refractory material and metal for cementing generally takes place in a temperature range of 1350.degree. to 1450.degree. C. which is an intermediate temperature between the melting point of the cobalt metal and the eutectic temperature of the cementing metal. In cermets of the TiC-Ni-Mo system, the eutectic temperature of the metallic components for cementing is about 1270.degree. C. and sintering usually takes place at less than 1455.degree. C. which is the melting point of nickel metal.
As mentioned above, when sintered bodies are produced, the sintering temperature is generally lower than the melting point of the cementing metal. In this case, the time required for the cementing metal to melt and for the transformation thereof into a liquid phase to occur at the sintering temperature in the heating process is governed by a change in the composition of the cementing metal due to diffusion in the solid state of the elements of the hard refractory material. Thus, the time required may vary depending on the manner in which the raw material powders are mixed with each other, the state of contact between the raw material powders, and the grain size of the cementing metal.